The next-generation mobile communication system aims to achieve a 1,000-fold increase in capacity, a 10-fold increase in spectral and energy efficiencies, and a 25-fold increase in average cell throughput. Such significant enhancements can be achieved with large-scale multiple-input multiple-output (MIMO) antenna systems, which are also referred to as massive MIMO systems. These systems employ hundreds, or even thousands, of antennas at base stations (BSs) to serve tens or hundreds of user terminals. As such, array gains are expected to grow infinitely with the number of antennas at the BSs, in which case the radiated energy efficiency increases dramatically, and multiuser interference is eliminated completely.
The main promises of massive MIMO are (i) simplified signal processing in the multi-user case, since simple conjugate beamforming provides quasi-optimum performance, and (ii) drastically reduced energy consumption, due to the high beamforming gain, and thus the possibility of lowering the transmit power while still retaining a high signal-to-noise ratio (SNR). However, these advantages are counteracted, in practice, by the increased hardware complexity associated with having many antennas and many associated up/down conversion chains, and by the increased energy consumption due to all that hardware.
One method for reducing MIMO complexity while retaining most of its benefits is antenna selection, where a subset of size L taken from the set of the N available antenna signals is selected and connected, via a switch, to L (L<N) radio-frequency (RF) chains. For the transmit case, each RF chain includes a modulator, digital-to-analog converter (DAC), and a power amplifier and each transmit antenna requires an RF chain; similar statements can be made for the receive case. This method has shown to provide the same diversity order as a full-complexity MIMO system. However, this method does not provide the same amount of beamforming gain, and thus shows reduced performance in particular in channels with small angular spread, as typically occur in cellular systems. See U.S. Pat. No. 9,705,579.
Accordingly, there is a need to share the hardware resources for beamforming transmission of a signal by an array of antennas.